Recently, I was tinkering around with an old air conditioning circuit board which was used to control a BLDC (Brushless Direct Current) motor for the fan when I found that tracing the circuits proved to be a bit more complicated than I thought. And my intention was just to make the motor work, rather than making the controls of the inverter air conditioning with all its speed control work. Furthermore, I was quite apprehensive when testing because of the high voltages involved.

So I decided to simplify the circuits by fabricating my own. In this way, I could follow the circuits just as I had fabricated them. I planned to use the existing electronic components and rewire them in a separate circuit board.

However, when I tried to de-solder the components out, I found that the heat from the soldering iron was not sufficiently hot to melt the existing solder. A 60 W soldering iron did managed to melt some of the solder, but unfortunately the former quickly became spoiled, perhaps due to overheating of its heating element.

Aluminium curtain rails

Then I read about people using hot air desoldering guns and how useful they were for desoldering, especially for surface mounted components like Integrated Circuits (IC). One interesting fact that I found was that nowadays, with modern circuit boards, there were often multiple layers to give more conducting surfaces for heat dissipation. This meant that the soldering iron that I had of about 40 W was not sufficient to bring the temperature of the solder to melting point. The heat simply dissipated away. This was very frustrating. Waiting and waiting but not able to melt the solder.

Bolt head embedded into a specially shaped head to fit the curtain rail

On reading further, I found that many desoldering rework stations had procedures that often had preheating prior to the actual desoldering. The circuit board to be reworked could be preheated in an oven or and a heating element could be placed underneath it. Recently, I encountered a similar problem while trying to repair a laptop recharger. I used a hair dryer as a preheating method and was successful in melting the solder. That got me thinking … A rework desoldering station would be useful for me.

Profiles of the curtain rail machined in plywood

So, off I went to my work bin and found some aluminium curtain rails that I had dismantled previously. I wanted to fashion them into an adjustable platform to place a circuit board for rework. The size should be large enough to place a large electronic board like a computer motherboard. The design should incorporate an easy-to-adjust way to cater for different sizes of boards.

Wooden knobs for tightening the bolts

The aluminium curtain rail was not flat, so some portions of it needed to be cut away. One side of the rail contained a groove and two curved edges. I planned to use the groove part to slide a clamping bolt and nut, but at the same time, I did not want to damage the edges of the rail when I clamped on it.

GI Gutter marked out for cutting

So, I designed a special piece to be attached to the bolt head so that the clamping forces on the aluminium rails would act against the curved plate faces instead of at its edges. I decided to use a soft material like plywood for this as it would be forced against the aluminium rail, which was also soft.

Almost finished heater box

Using my movable bed cnc machine, I was able to cut the shapes out of a 12 mm plywood using a 1/8″ end mill and glued two halves of them together to form a sliding part bolt head. At first I wanted to make the height of the platform to place the circuit board adjustable, with the same concept of clamping them in position. However, after thinking about it for a while, I decided that it would be too difficult to make the structure stiff – I would need some right-angled brackets in order to make them hold their shape. Instead of legs, I decided to use two pieces of phenolic boards and bolted them to the aluminium curtain rail. In this way, I was able to create a platform to rest the circuit board, just by adjusting the width and tightening the clamping nuts.

I also fabricated some knobs out of the same 12 mm plywood and was able to use these for tightening the clamping nuts. As you can see from the video below, the soldering station was able to hold a large circuit board in place.

Incidentally, I also designed and fabricated a crude incandescent light bulb heater out of a discarded rain water gutter down-pipe to aid in my de-soldering work. It did not work as well as I thought, as the heating was too slow for my very first job, which was to replace 23 pieces of bulging capacitors from the motherboard of a computer. Luckily the soldering iron was hot enough without needing additional heating.

The Movable Gantry CNC Machine will have a much larger cutting area than my Movable Bed CNC Machine. So this would, in fact, give me more flexibility in terms of the jobs that I could handle. What I could think of at this time was the use of a drag knife or a tangential knife for cutting thin sheets of cardboard, vinyl, cloth or other similar materials. I would certainly want to try this out when my movable gantry cnc machine was ready.

Gathering information from the internet, I came to the conclusion that there were some distinct differences on how these knives work. The drag knife turns on its own while the tangential knife had to use a stepper motor and programming to turn to the required angle. Since the tangential knife was more complicated, I would not want to delve into it at this moment.

The drag knife worked like the back rollers of a supermarket trolley. The rolling action of the back wheels, being off-centered from the side thrust turning axis, was able to make the wheels rotate in the direction of the side thrust. However, just like the supermarket trolley, there would be a certain position of the wheels when the wheels would not behave as required.

In the drag knife, if the blade were to be caught immovable deep inside the material to be cut and is then required to change direction, there was a likely chance that the blade would break. In order to avoid this phenomenon, the software that generated the gcode had to add in some additional codes before changing direction so that the knife would move up very near to the surface of the material, change the direction, plunge deeper back again, and then proceed with the normal gcode movements.

Since the direction of the drag knife was dependent on the movement of the machine, the knife blade must always touch the material to be cut when the movement was made. Careful selection of the contact depth of the knife blade when changing direction was essential.

The parts of the drag knife was simple. Essentially, it consisted of a vertical shaft with a means to attach an offset knife blade, being free to rotate on ball bearings following the movement of the cnc machine. I have looked at how commercially available drag knifes work but they were too expensive for a hobbyist like me. The Grunblau drag knife available as a plan seemed a good option, but since it still required some aluminium machining-fabrication from me, I decided to design and fabricate my own design.

You can call it the “Yoon Drag Knife”. The essential components of a drag knife was still there, but I felt that making the moving parts out of steel would suit me better since I wanted a strong material with a minimum of machining. I also had an arc welding machine which I felt could be used to make reasonably strong joints. Any other non-stressed features like locators for the knife could be done using metal strips glued by epoxy resin. Rough edges, arc spatter or other visual defects could be cleaned up by a hand grinder or file and any holes could be drilled using my portable drilling machine.

The Yoon Drag Knife was designed to fit in any normal 60 degree angled blade bought from the local hardware store. The shaft was a short length of 8 mm diameter stud which was ground flat at one side in order to coincide with the center line of the knife blade when the blade holder was in place. These were welded in place. The whole assembly would be placed in the bearings and held in place by nuts.

The ball bearings would be clamped in place by bearing housings machined out from 11 mm plywood by nuts and bolts. These would be the only machining that I needed to do on my movable bed cnc machine. Separately, the complete bearing housing unit would then be secured to the z-axis spindle holder of the movable gantry cnc machine by nuts and bolts. Thus, I eliminated the use of a shaft at the opposite end of the drag knife which other designs had, because I felt that this was a weak point. I did not have a lathe to ensure the centering of the shaft was true. When in used, I just needed to remove the spindle and replace it with the drag knife assembly.